The present invention relates to measuring techniques, more particularly to a method of determining geometric parameters of an object's surface and a device therefor. It may be used to advantage in determining shape and deformation of objects and for measuring the path of a moving object, for example determining the deformation and path of blades of a helicopter rotor.
Besides, the invention may find wide applications in precision determination of a third coordinate of surface with two known quantities as is often the case in machine building and various measurements, particularly in noncontact measurements.
An extremely difficult problem of current interest is the noncontact determination of geometric parameters of objects' surfaces such, for example, as surfaces of various elements of flight vehicles, ships, machine structures and water surfaces, as well as determination of shape, position, movement and deformation of such surfaces. Known in the art is a great number of noncontact optical methods and devices based on holographic, moire' and light interference principles designed to meet specific requirements relating to the above problem.
Known in the art is a method of determining geometric parameters of an object's surface (cf. U.S. Pat. No. 3,619,064, Cl. 356/109 "Moire' Measuring System" granted to Robert Brookes on Nov. 9, 1971).
The aforesaid method comprises the steps of producing an interference pattern on a surface under investigation, projecting thereon at an angle with respect to each other two beams of coherent light, recording said interference pattern, producing thereafter the same interference pattern on the object's surface at another moment or on the surface of another object, aligning the latter pattern with the one recorded previously and determining a change in the shape of the object's surface or a difference between shapes of surfaces of the two objects being compared.
However, the moire' method permits determining only a difference in the shape of the object's surface at different moments or a difference in the shapes of the surfaces of two objects, which can be utilized for measuring deformation of the object or for comparing the objects with a reference. A disadvantage of the aforesaid method is that actual or true shape and attitude of the object surface may not be determined.
Furthermore, the moire' method when used for determining deformation for comparing with a reference introduces a considerable error equalling half a period of interference bands with a comparatively small variation of geometric parameters due to the inherent condition that the period of moire' bands should exceed by several times the period of an interference pattern on the surface under investigation.
Also known in the art is a method of determining geometric parameters of an object (cf. U.S. Pat. No. 3,907,438, Cl. 356/156 "System for Measuring Cylinder Loop" granted to John Holeman on Sep. 23, 1975). With such a method, an interference pattern is produced on the surface of an object under investigation, said pattern being subsequently recorded, while desired geometric parameters are determined with due regard for the number and shape of interference bands on the recorded image.
With two intersecting beams of coherent light projected onto the surface at a non-zero incidence angle, interference patterns are formed on the surface, which represent lines of intersection of the surface under investigation and the surfaces of interference extremums in space.
Thus, the interference bands on the surface under investigation represent topographic level contours set by the surfaces of interference extremums. Knowing the angle between the beams .alpha. it is possible to determine the period of the interference extremums S by the formula ##EQU1## where .lambda. is light wavelength.
Determining the charging number of the interference band .DELTA. n at each point set by two coordinates, X and Y, on the surface with respect to another (reference) point and knowing the period of the interference extremums S and the angle of incidence of beams on the surface .theta. it is possible to find relative deviation of a given point on the surface on the third coordinate .DELTA. Z: ##EQU2##
Measuring a change in the band number allows determining integral and fractional parts of an interval between the deviation bands. Therefore, the measuring accuracy of the aforesaid method is higher than that in the case of the moire' method.
Moreover, no limitation is imposed on the range of measured deviations of the surface, which is an apparent advantage over the moire' method.
However, the two prior art methods make it possible to determine only relative deviation of shape of the surface under investigation from the original or reference surface, a serious disadvantage being that true shape, attitude and movement of the surface may not be determined.
Another disadvantage of the known method is the need for producing and projecting on the investigated surface two collimated light beams with values S and .theta. being constant in the entire intersection area. Thus, the section of collimated beams determines the size of surfaces under investigation. This excludes the possibility of investigating real large objects such, for instance, as airfoils of flight vehicles.
If the surface under investigation is struck by two diverging beams of coherent light, the values of parameters S and .theta. will be different at different points in the area of intersection of the beams. In this case problems of determining relative changes in the shape of the surface may not be properly solved unless prior information on displacement of at least one point on the surface is available.
One more disadvantage of the aforesaid method is that the number of the band on the recorded pattern is determined either by visual counting or by photometric evaluation of the image and the counting of maximum and minimum blackening densities.
In the former case there may be considerable subjective errors, while in the latter case the occurrence of counting errors may be the result, said counting errors being attributed to spurious maximum and minimum image blackening densities due to non-uniformity of reflectivity of the investigated surface, presence of small elements, irregularities and defects thereon, porosity or granular structure of photosensitive material used for image recording, and other possible causes such as image defects occurring in recording, etc. An apparent disadvantage of the foregoing method is, therefore, inadequate accuracy and reliability of measurements.
A known device for determining geometric parameters of an object's surface comprises an optical system for producing two beams of coherent light and projecting said beams on the surface under investigation, a photorecorder for recording an interference pattern on said surface, and an assembly for measuring coordinates and the number of bands of the interference pattern (cf. U.S. Pat. No. 3,907,438). To illuminate surfaces of large objects, the optical system may include means for producing two diverging beams of coherent light (cf. U.S. Pat. No. 3,619,064, Cl. 356/109).
However, the known device permits determining only the relative increment of the coordinate at a given point with respect to the coordinate at other points on the surface. Thus, the device does not respond to general movements of the object's surface and to its shape. In the case of large objects the known device allows determininig relative movements only when additional information on displacement of at least one point on the surface is obtained from other sources. In other instances the device may be used only to obtain precise data on geometric parameters of the surface under investigation.
Another disadvantage of the aforesaid device is that the known construction of the unit for measuring the number of bands of the recorded interference pattern does not ensure adequate accuracy and reliability of measurements due to the existance of a subjective factor in the process and also because of inherent sensitivity to defects and secondary elements of the recorded image and photorecording material.
Furthermore, such construction makes it impossible to automatically analyze the recorded pattern primarily due to high probability of spurious counts and low noise immunity. Therefore, the analysis of images with a view to obtaining precise data on geometric parameters entails an intricate, long and labour-consuming process.
It is an object of the present invention to provide a method for the noncontact determination of basic geometric parameters of object's surface such as shape, attitude, motion, and deformation.
Another object of the invention is to provide a method for the noncontact determination of said geometric parameters without using any prior information on motion and shape of objects.
One more object of the invention is to provide a method and a noncontact measuring device therefor possessing high precision, reliability, and repeatability of results.
A still another object of the invention is to provide a noncontact measuring device for executing said method, which is characterized by automatic measuring cycle and data processing without any subjective elements.